1. Field of the Invention
The present invention relates to a thermally-assisted magnetic recording head for use in thermally-assisted magnetic recording in which a recording medium is irradiated with near-field light to lower the coercivity of the recording medium for data writing.
2. Description of the Related Art
Recently, magnetic recording devices such as magnetic disk drives have been improved in recording density, and thin-film magnetic heads and recording media of improved performance have been demanded accordingly. Among the thin-film magnetic heads, a composite thin-film magnetic head has been used widely. The composite thin-film magnetic head has such a structure that a read head unit including a magnetoresistive element (hereinafter, also referred to as MR element) for reading and a write head unit including an induction-type electromagnetic transducer for writing are stacked on a substrate. In a magnetic disk drive, the thin-film magnetic head is mounted on a slider that flies slightly above the surface of a recording medium. The slider has a medium facing surface facing the recording medium. The medium facing surface has an air inflow end (a leading end) and an air outflow end (a trailing end).
Here, the side of the positions closer to the leading end relative to a reference position will be defined as the leading side, and the side of the positions closer to the trailing end relative to the reference position will be defined as the trailing side. The leading side is the rear side in the direction of travel of the recording medium relative to the slider. The trailing side is the front side in the direction of travel of the recording medium relative to the slider.
To increase the recording density of a magnetic recording device, it is effective to make the magnetic fine particles of the recording medium smaller. Making the magnetic fine particles smaller, however, causes the problem that the magnetic fine particles drop in the thermal stability of magnetization. To solve this problem, it is effective to increase the anisotropic energy of the magnetic fine particles. However, increasing the anisotropic energy of the magnetic fine particles leads to an increase in coercivity of the recording medium, and this makes it difficult to perform data writing with existing magnetic heads.
To solve the foregoing problems, there has been proposed a technology so-called thermally-assisted magnetic recording. The technology uses a recording medium having high coercivity. When writing data, a write magnetic field and heat are simultaneously applied to the area of the recording medium where to write data, so that the area rises in temperature and drops in coercivity for data writing. The area where data is written subsequently falls in temperature and rises in coercivity to increase the thermal stability of magnetization. Hereinafter, a magnetic head for use in thermally-assisted magnetic recording will be referred to as a thermally-assisted magnetic recording head.
In thermally-assisted magnetic recording, near-field light is typically used as a means for applying heat to the recording medium. A known method for generating near-field light is to use a plasmon generator, which is a piece of metal that generates near-field light from plasmons excited by irradiation with laser light. The laser light to be used for generating near-field light is typically guided through a waveguide, which is provided in the slider, to the plasmon generator disposed near the medium facing surface of the slider.
U.S. Patent Application Publication No. 2011/0058272 A1 discloses a technology in which the surface of the core of the waveguide and the surface of the plasmon generator are arranged to face each other with a gap therebetween, so that evanescent light that occurs from the surface of the core based on the light propagating through the core is used to excite surface plasmons on the plasmon generator to generate near-field light based on the excited surface plasmons.
A thermally-assisted magnetic recording head that employs a plasmon generator as a source of generation of near-field light is configured so that the write head unit includes a coil, a main pole, and the plasmon generator. The coil produces a magnetic field corresponding to data to be written on a recording medium. The main pole has an end face located in the medium facing surface. The main pole allows a magnetic flux corresponding to the magnetic field produced by the coil to pass, and produces a write magnetic field from the aforementioned end face. The plasmon generator includes a near-field light generating part located in the medium facing surface. For the thermally-assisted magnetic recording head, it is demanded that the end face of the main pole and the near-field light generating part of the plasmon generator be located in close proximity to each other.
To increase the linear recording density of a magnetic recording device, it is effective to use a perpendicular magnetic recording system in which the direction of magnetization of signals to be written on tracks of a recording medium is perpendicular to the plane of the recording medium. It is also effective to increase, on the tracks, the gradient of the change in write magnetic field intensity with respect to the change in position along the direction in which the tracks extend, i.e., the direction along the tracks (this gradient will hereinafter be referred to as the write field intensity gradient). These also apply to a magnetic recording device that employs thermally-assisted magnetic recording.
In order to increase the write field intensity gradient in a magnetic head of the perpendicular magnetic recording system, it is effective to provide a shield that has an end face located in the medium facing surface at a position near the end face of the main pole. U.S. Patent Application Publication No. 2011/0058272 A1 discloses a technology for increasing the write field intensity gradient by providing a bottom shield on the leading side of the main pole, the bottom shield having an end face located in the medium facing surface.
A magnetic head including a shield is typically provided with a return path section for connecting the shield to a portion of the main pole located away from the medium facing surface. One or more spaces are defined between the return path section and the main pole. The coil is provided to pass through the one or more spaces.
Now, consider a thermally-assisted magnetic recording head configured so that the near-field light generating part of the plasmon generator is interposed between the end face of the main pole and the end face of the shield, and the core of the waveguide and the return path section intersect each other without contacting each other. A general approach to precluding the contact between the core and the return path section is to branch a portion of the return path section that intersects the core into two portions so as to detour around the core and then merge the two portions into one, as disclosed in U.S. Patent Application Publication No. 2011/0058272 A1. When this approach is employed, the return path section is formed to include a coupling portion for coupling the two branched portions. The coil is wound around the coupling portion, for example.
In the above-described configuration, the width of the coupling portion in the track width direction is equal to or greater than the distance between the respective outer ends of the two branched portions in the track width direction, thus being comparatively great. The coil should be long in entire length if wound around the coupling portion. In this case, the coil has a high resistance, and consequently a high heating value. This gives rise to a problem that components around the coil expand and as a result, part of the medium facing surface protrudes toward the recording medium and may readily collide with the recording medium. In order to prevent this, the distance between the medium facing surface and the recording medium could be increased. However, this would disadvantageously lead to deterioration in write characteristics such as the overwrite property or to an increase in error rate.
On the other hand, in order to improve the write characteristics in a high frequency band, it is desirable that the main pole, the shield and the return path section should form a magnetic path of reduced length. To achieve this, it is effective to bring the portion of the return path section intersecting the core into closer proximity to the medium facing surface. Here, assume that the coil is wound around the coupling portion of the return path section. In this case, since the coupling portion is comparatively great in width in the track width direction as mentioned above, the coil should include one or more conductor portions located between the coupling portion and the medium facing surface and extending linearly in parallel to the medium facing surface (such one or more conductor portions will hereinafter be referred to as linear conductor portion(s)). Bringing the portion of the return path section intersecting the core into closer proximity to the medium facing surface causes the linear conductor portion(s) to be narrow and long. This in turn causes the coil to be higher in resistance, so that the above-described various problems will become more noticeable. Accordingly, in this case, it is difficult to reduce the length of the magnetic path formed by the main pole, the shield and the return path section.
Further, a thermally-assisted magnetic recording head has the following problems. Part of the energy of light guided to the plasmon generator through the waveguide is transformed into heat in the plasmon generator. Part of the energy of near-field light generated by the plasmon generator is also transformed into heat in the plasmon generator. Thus, during the operation of the thermally-assisted magnetic recording head, the plasmon generator rises in temperature and expands to protrude significantly toward the recording medium. As a result, in the thermally-assisted magnetic recording head, a protective film covering the medium facing surface may be broken at a portion near the plasmon generator by contact with the recording medium. If the portion of the protective film near the plasmon generator is broken, the plasmon generator may be damaged by contact with the recording medium or may be corroded by contact with high temperature air.